The step called generally photolithography is needed for the formation of circuit patterns for semiconductor devices. For this step, usually, there is a transfer process used, which transfers a photomask (often called the reticle, and hereinafter called simply the “mask” pattern onto the alignment substrate such as a semiconductor wafer. The alignment substrate is coated with a photosensitive photoresist, and a circuit pattern is transferred onto the photoresist depending on the pattern shape of the mask pattern. And at a projection aligner, the image of the circuit pattern written on the mask and to be transferred is projected onto the alignment substrate (wafer) and exposed to light by way of a projection optical system.
In photolithography, the minimum size (resolution) that can be transferred with the projection aligner is proportional to the wavelength of light used for exposure yet in inverse proportion to the numerical aperture (NA) of a projection optical system; so with a growing demand for recent finer and finer semiconductor devices, exposure light wavelengths are growing shorter and the NA of the projection optical system is growing larger. Only by use of such shorter wavelength and larger NA, there is still a limit to meeting this demand.
To bring up resolution, there has recently been ultra-resolution technologies proposed, according to which the value of a process constant kl (kl=resolution line width×numerical aperture of the projection optical system×wavelength of exposure light) is made small enough to achieve ever finer patterning. Such ultra-resolution technologies, for instance, include an optimization method of giving an auxiliary pattern or line width offset to a mask pattern depending on the characteristics of an aligner optical system, and a method called the modified illumination method (also called the oblique incidence illumination or multipole illumination method). For the modified illumination method, for instance, dipole illumination, and quadrapole illumination is used.
FIG. 9 is illustrative in schematic of the general setup of an aligner system in a semiconductor-specific aligner, which comprises a light source 91 such as an ArF excimer laser and a pupil filter 91 working as a modified illumination means. Illumination light 93 is imaged on a wafer 98 of an alignment substrate by way of a lens 97 that defines a projection optical system. The “pupil filter” here refers to the pupil filter of an illumination optical system that is located on a condenser lens 94 on a mask 95.
FIG. 10 is a schematic top view illustrative of one exemplary configuration of a conventional pupil filter comprising a hatched area defining a light block area and a blank area defining a light transmissive area. More specifically, FIG. 10(a) is illustrative of an ordinary pupil filter in which most of the round pupil defines a light transmissive area 101 through which light emanating from a light source transmits, with its peripheral area defining a light block area 102. FIG. 10(b) is illustrative of an exemplary pupil filter configuration for modified illumination, in which the central area defines a light block area 104 and two fan-form light transmissive areas 103 are provided at positions symmetric about the center of the pupil filter.
With such a general round pupil filter as depicted in FIG. 10(a), the angle of diffraction (t) of diffracted light from the mask is determined by the pattern pitch (d) of the mask and the wavelength (L) of light, as given by the following equation (here n is the degree of diffraction). Accordingly, when a specific light source is used, the angle of diffraction (t) of diffracted light is going to differ depending on the pitch of the mask pattern.sin(t)=n×L/d 
FIG. 11 is illustrative of the relations of the pitch of the mask pattern to diffracted light of illumination light after having passed through the pupil filter.
FIG. 11(a) is illustrative of the case where the pattern pitch of a mask 115a is large relative to the exposure light wavelength, showing that illumination light 113a is imaged on a wafer surface, forming a good image. However, as semiconductor devices get finer and finer, mask pattern pitches get smaller and smaller, too. FIG. 11(b) is illustrative of the case where the pattern pitch of a mask 115b is small relative to exposure light wavelength, showing that illumination light 113b is not imaged on a wafer surface, failing to give a good image. FIG. 11(c) is illustrative of oblique incidence of light under the same conditions as in FIG. 11(b)—oblique incidence illumination method, showing that illumination light 113c can be imaged on a wafer. Thus, as semiconductor devices grow finer, mask pattern pitches get smaller; so oblique incidence illumination techniques are now in use.
If, to put to practice the oblique incidence illumination technique shown in FIG. 11(c), the pupil shape of the pupil filter is changed from the conventional round pupil shown in FIG. 10(a) to the dipole fan-form pupil of FIG. 10(b) for instance, the middle of the pupil filter could be blocked against light so that light of an oblique component could be incident on it, thereby improving resolving power.
However, the changing of the illumination pupil filter from FIG. 10(a) to FIG. 10(b) is found to engender other problems.
One problem comes to arise when the pupil filter of FIG. 10(b) is achieved by an aperture (generally metal sheet machining); the quantity of light transmitting through the pupil filter of FIG. 10(b) diminishes or much time is taken for exposure, making the efficiency of semiconductor exposure much worse and, hence, semiconductor fabrication cost much. So far, the use of a diffractive optical device has been proposed to minimize losses of the aforesaid quantity of light (for instance, see Patent Publications 1 and 2).
Because the pupil filter of FIG. 10(b) is smaller than the pupil filter of FIG. 10(a) in terms of the area through which illumination light transmits, light leaving the pupil filter becomes light of very strong coherence. This gives rise to another problem.
FIG. 12 is a schematic top view of a pattern indicative of a mask pattern pitch, with a black area defining a light block area; FIG. 12(a) illustrates a small pitch and FIG. 12(b) illustrates a large pitch. FIG. 13, with a mask pattern pitch as abscissa and an optical image contrast as ordinate, is illustrative of the relations of the mask pattern pitch to the coherence of illumination light leaving the pupil filter. In FIG. 13, a curve (a) is about a round pupil and a curve (b) is about a fan-form pupil.
As the mask pattern pitch becomes small and narrow on the mask, it causes the contrast to go down starkly and the resolving power to drop under ordinary illumination using the round pupil, as shown by the curve (a) in FIG. 13. With a large and wide pitch, however, there is the contrast staying stable.
As shown by the curve (b) in FIG. 13, on the other hand, modified illumination using the fan-form pupil keeps the contrast high to where the pitch is small. As the pattern pitch varies, however, it causes the contrast to change regularly or become erratic. This means that there is a fluctuation in the size of a pattern imaged on the wafer due to the mask pattern pitch. To make correction of that fluctuation, powerful correction techniques such as OPC (optical proximity correct) are in need. However, illumination light leaving the pupil filter is of high coherence; that is, there is a wider extent to which OPC must be applied, which imposes too much load to data processing, giving rise to a problem: correction by OPC is difficult, if not impossible.
Patent Publication 1: JP(A)2001-174615
Patent Publication 2: JP(A)2005-243953